Project 3 addresses the development of MHC class II restricted presentation function and its modulation by HCMV. Developmental immaturity of professional antigen presenting cells in young children may underlie the poor induction of an adaptive immune response and result in long-term shedding of HCMV that is characteristic of primary infection in these individuals. Furthermore, HCMV infection of antigen presenting cells and their progenitors may hinder antigen presentation. As a result, cells in the myeloid dendritic lineage that are productively or latently infected in the induction of an effective adaptive immune response may be delayed. The first aim will be to compare the composition and function of APC populations in peripheral blood of neonates, young and function of cell from neonates and infants will be compared to those found in older children and adults, establishing profiles in HCMV infected as well as non-infected individuals. The phenotype and function of cells from neonates and infants will be compared to those found in older children and adults using multiparameter flow cytometry of cell subsets, activation markers and cytokine expression. The phenotype and function of cells from neonates and infants will be compared to those found in older children and adults using multiparameter flow cytometry of cell subsets, activation markers and cytokine expression patterns. The second of virus infection during primary infection. Both antibody and nucleic acid-based in situ detection methods will be used to assess the distribution of virus over a six month observation period. Qualitative and quantitative aspects of viral gene expression will be evaluated by in situ antigen detection, RT-PCR and RT-PCR-enhanced in situ hybridization. These data will provide the first detailed information of viral gene expression in leukocytes during primary infection and will allow the parameters of viral acute infection to be related to both immune clearance and developmental status, as well as the transition to latent infection. The third aim will investigate the impact of cytomegalovirus infection on class II biosynthesis, peptide loading and trafficking in both productively infected mature dendritic cells as well as latently infected progenitors that express reduced levels of cell surface MHC class II and will identify subset(s) of cells that are modulated. Together, this project will bring a new level of information on the nature of developing immune response in young children deduced in the context of natural infection by this important pathogen. The information will lead directly to better means of evaluating efficacy of vaccines intended to control cytomegalovirus and other infectious disease agents.